Characteristics of IncX1 Plasmids encoding tigecycline resistance gene tet(X4) and their bacterial hosts in healthy service workers

Objective To understand the genomic characteristics of IncX1 plasmids carrying tigecycline resistance gene tet(X4) isolated from healthy service workers and the global distribution of IncX1 plasmids.

Methods  Systematic stool sample collection was conducted in healthy service workers in Shandong province and Guangxi Zhuang autonomous region. The strains carrying the tet(X4) gene and IncX1 plasmids were screened by using tigecycline-containing plates and fluorescent polymerase chain reaction. Antimicrobial susceptibility testing was performed by using broth microdilution method. Whole-genome sequences of the strains and their plasmids were obtained by the combination of second and third generation sequencing. The global distribution of IncX1 plasmids was analyzed using the plasmids in pathogens database (PIPdb). Comparative genomics, phylogenetic analysis, and genetic structure alignment were used to reveal the evolutionary relationships and genetic features between the plasmids identified in this study and those in public databases. Conjugation experiments were conducted to determine plasmid transfer efficiency.

Results Based on complete genome sequence analysis, two IncX1 plasmids carrying tet(X4) gene were identified. The core genetic context of tet(X4) in both plasmids was IS26-rdmC-tet(X4)-ISCR2, which can carry resistance genes tet(A), floR, linG and ANT(3'')-IIa stably. Conjugation experiments confirmed that the plasmids were transferable and could mediate tigecycline resistance.

Conclusion  IncX1 plasmids carrying tet(X4) gene were detected in healthy service workers, suggesting the potential risk for community transmission. It is suggested to include healthy service workers in routine antimicrobial resistance surveillance and establish a resistance plasmid monitoring system based on complete plasmid genome sequences.